In the operation of air conditioning systems and many types of refrigeration systems, the electric motor that is used to drive the refrigeration compressor is located remotely from an area that would normally be monitored or occupied. In this type of a situation, a motor failure, such as a stalled motor or a motor with an open winding, can go undetected for quite a long period of time. In refrigeration equipment used for cooling and storage of food and other perishables, the long interval of time between a compressor motor failure and the realization that such a failure has occurred can be very costly. In the failure of air conditioning equipment, the failure to promptly service equipment can lead to considerable inconvenience.
In many air conditioning and refrigeration systems it has been found possible to connect an electromagnetically operated relay across the run capacitor of a permanently split capacitor run type of motor for use in indicator or fault detection equipment. If an electromagnetically operated relay is connected across the run capacitor, a voltage change that appears across this capacitor during the normal run condition and a fault condition can be used in some installations to indicate the existence of a fault or improper operation. The difficulty in using a relay is that while the relay will pull-in at a well defined voltage, the dropout voltage of a conventional electromagnetic relay creates a problem in possibly not indicating one type of compressor motor fault. If the run winding of a permanently split capacitor motor that is being monitored by a relay opens while the motor is in operation, a voltage across the run capacitor will result from the resonant circuit formed by the run capacitor and the starting winding. If a relay is used as a detector in this type of a situation, the resonant voltage will not allow the relay to drop out, therefore, not indicating a fault. The wide differential in the pull-in and dropout characteristics of a conventional electromagnetic type relay, therefore, makes its use of limited value.